This invention relates to drive circuitry for a cathode ray tube.
The use of the cathode ray tube for the display of computer-generated data has led to more demanding operating standards being required of the drive circuitry as compared with the requirements of domestic television equipment. Many of the circuits which have proved adequate for television have had to be redesigned. Although much research is being put into alternative display technologies, the low cost and continued improvements of the cathode ray tube have ensured that efforts continue to be made to improve its capacity and resolution. It is envisaged that the cathode ray tube can display images consisting of over four million picture elements (pels) produced by the momentary brightening of the cathode ray beam as it is caused to scan a raster across the tube screen. Such a requirement implies a line frequency of about 100 kHz, which would impose extremely high stresses on the active device in conventional circuitry which relies on sawtooth wave generators. A high line frequency means a short flyback time during which stored energy must be removed from the deflection coils. Because of imperfect switching performance at these power levels, the peak collector dissipation in the line scan transistor can reach 1 Kw with an average value of about 50 w. Such stresses will lead to component failure and consequent device unreliability. Providing suitable higher power components raises the costs of the drive circuitry to unacceptable levels and means that advantage cannot be taken of integrated circuit techniques.
In the past, much effort has been expended in obtaining good linearity of the line scan, that is to ensure that the beam moves along each scan line at a constant speed. This permits the use of a constant frequency clock to control the modulation of the beam intensity. Variations in scanning speed cause images to be distorted due to the beam being turned on at the wrong position.
To avoid the stresses involved with a conventional sawtooth line scan circuit, it may be desirable to use a sinusoidal line scan generator and/or to use a bidirectional raster. A bidirectional raster is described, for example, in the IBM Technical Disclosure Bulletin, Volume 21, No. 12, May 1979 at pages 5025 and 5026. A sinusoidal deflection waveform is described, for example, in the IBM Technical Disclosure Bulletin, Volume 14, No. 4, September 1971 at pages 1118 and 1119. One problem with such techniques, however, is how to achieve adequate linearity of the line scan. A second problem is present when bidirectional scanning is employed and the ferrite core of the deflection yoke exhibits hysteresis or the drive waveform contains asymmetric distortion components. These aberrations cause vertical misalignment of the pels in alternate scan lines.
Although it is highly desirable for the line scan to be perfectly linear, it is unlikely, or indeed impossible that exact linearity can be obtained in practice. Clearly non-linearity will be a problem in any high precision CRT display, whether it employs standard sawtooth, modified sawtooth or sinusoidal line scan waveforms with or without bidirectional raster scanning.